Zhuojin Pipe Fitting Co., LtdFinite Element Strength Analysis of Large-Diameter Equal-Thickness Eccentric Welded Tee
Literature Overview
The paper by Li Meng-li, Ma Xiao-hui, Shang Xian-dong, and colleagues (2021), published in the Journal of Qilu University of Technology, presents a finite element analysis (FEA) of a large-diameter equal-thickness eccentric welded tee under combined internal pressure and external load conditions. Funded by the National Natural Science Foundation Youth Science Fund (51705265) and the National Key R&D Program (2016YFC0802303), this work draws on expertise from Qilu University of Technology, Shandong Hengli New Energy Engineering, Shandong Special Equipment Inspection Research Institute, and Linde Air Products (Hangzhou). The study is directly relevant to pressure vessel and piping engineers who must evaluate the structural integrity of tee fittings under complex loading.
Modeling and Analysis Methodology
The authors employed ANSYS Workbench to construct a finite element model of the eccentric welded tee and performed stress analysis under both design and operating load conditions. The analysis followed the stress analysis methods specified in the ASME Code, classifying stress categories and stress combinations according to code requirements.
| Load Condition | Maximum Equivalent Stress (MPa) | Location of Maximum Stress |
|---|---|---|
| Design load | 234.89 | Outer belly (pipe opening to the left) |
| Operating load | 228.56 | Shoulder acute angle at the intersection area |
Both maximum stress values exceeded the material's design stress intensity, identifying high-stress regions that require further evaluation.
Stress Evaluation and Code Compliance
To determine the structural safety under pressure-bearing conditions, the authors applied the evaluation criteria specified in JB 4732-1995 (Steel Pressure Vessels - Analytical Design), comparing calculated stress values against allowable material stress values. The high-stress regions were subjected to strength verification through stress linearization at selected evaluation locations.
The stress linearization process separates the through-thickness stress distribution into membrane (average) stress, bending (linear) stress, and peak (localized) stress components. Each component is then compared against its respective allowable limit:
| Stress Category | Allowable Limit Basis | Verification Result |
|---|---|---|
| Membrane stress | 1.0 × S (design), 1.5 × S (operating) | Satisfied |
| Bending stress | 1.5 × S (design), 2.0 × S (operating) | Satisfied |
| Peak stress | 3.0 × S (design), 4.0 × S (operating) | Satisfied |
The analysis results satisfied the code requirements under both design and operating conditions, confirming that the eccentric welded tee structure is safe for the intended service.
Engineering Practice Insights
The eccentric tee is a common fitting in piping systems where the centerline of the branch pipe does not coincide with the centerline of the run pipe. This geometric asymmetry creates complex stress concentrations that are difficult to evaluate analytically. The FEA approach demonstrated in this paper provides a practical methodology for code-compliant stress evaluation.
Several practical observations emerge from this study:
- The maximum stress under design load occurred at the outer belly of the tee, which is the region farthest from the branch pipe opening. This is consistent with the bending moment effects of the eccentricity.
- Under operating load, the maximum stress shifted to the shoulder acute angle at the intersection area, reflecting the change in load combination and the stress concentration effect of the geometric discontinuity.
- The stress linearization method is essential for distinguishing between primary membrane stress (which must be limited to prevent excessive deformation) and secondary stress (which can be higher due to self-limiting behavior).
Key Questions and Reflections
The study raises several important considerations for engineering practice:
- How sensitive are the stress results to mesh density and element type? A convergence study would strengthen the credibility of the FEA results.
- What is the effect of the weld geometry and weld reinforcement profile on the stress distribution? The model appears to treat the tee as a seamless structure, which may not reflect the actual weld geometry.
- Should fatigue analysis be considered for cyclic loading conditions? The operating load condition suggests that the tee may be subjected to repeated pressure cycling, which could lead to low-cycle fatigue at the high-stress locations.
Summary and Conclusions
This paper demonstrates a rigorous and code-compliant approach to the structural evaluation of eccentric welded tees using finite element analysis. The methodology of stress classification, linearization, and comparison against code allowable limits provides a clear framework that can be applied to other complex fitting geometries. For engineers involved in the design and inspection of pressure piping systems, this study reinforces the importance of FEA in evaluating fittings where analytical methods are insufficient. The key practical takeaway is that even when maximum equivalent stresses exceed the material design stress, the structure may still be acceptable under code rules if the stress linearization shows that the primary membrane stress is within limits.